The United States has condemned the "senseless and inhumane" extremist attacks in Pakistan after a day of bomb blasts that killed 126 people and sparked angry protests by Shiite Muslims.

Two suicide bombers killed 92 people and wounded 121 when they targeted a crowded snooker club in the south western city of Quetta on Thursday, in an area dominated by Shiite Muslims from the Hazara ethnic minority.

US State Department spokeswoman, Victoria Nuland, on Friday said Washington offered "the families of those killed in these brutal attacks in Quetta, in Swat, in Karachi... our deepest condolences".

"We obviously stand with the people of Pakistan in strongly condemning these senseless and inhumane acts," she added.

"We remain concerned about extremist violence of any kind in Pakistan and remain committed to working with the government of Pakistan to combat terror."

Extremist Sunni militant group Lashkar-e-Jhangvi claimed responsibility for what was the worst ever sectarian attack on Shiites, who account for around 20 per cent of Pakistan's 180 million population.

Hundreds of Shiites staged a sit-in at the devastated snooker hall on Friday, refusing to bury loved ones until the army takes responsibility for security in Quetta from paramilitary and police officers.

The government in Baluchistan province, which is also fighting a separatist insurgency, announced three days of mourning, but the protesters squatted on the road alongside around 30 bodies draped in shrouds and placed in coffins.

It was the deadliest attack in Pakistan since suicide bombers killed 98 people outside a police training centre in the north west in 2011 - an assault claimed by the Pakistani Taliban.

Meanwhile, unidentified men fired rockets on NATO containers carrying goods for international troops operating in Afghanistan, killing one person and destroying at least five vehicles in the Hazar Ganji area on the outskirts of Quetta on Friday.

"A group of at least 12 unidentified men attacked the terminal and fired rockets, which triggered a massive fire and gutted five out of 10 containers parked there," senior local police official Hamid Shakeel said.

He said that the attackers escaped after also shooting dead an employee at a nearby petrol pump.

A local police official, Noor Bukhsh Mengal, also confirmed the incident.

Earlier on Thursday, a bomb detonated under a security force vehicle in a crowded part of Quetta, killing 11 people and wounding dozens.

A bomb at a religious gathering in the north western Swat valley killed 22 people and wounded more than 80, the deadliest incident in the district since the army in 2009 fought off a two-year Taliban insurgency.

The attacks, coupled with violence in the north west, revived warnings from analysts that Islamist militancy could threaten national elections, expected some time in May.

The Human Rights Commission of Pakistan demanded that the government take immediate steps to clamp down on "murdering mayhem" ahead of the elections.